The Florida Everglades, an extremely diverse ecosystem that hosts a wide variety of flora and fauna, began to form about 5000 years ago when sea levels stabilized after the last ice age. Mangrove coastlines, hardwood forests, and cypress swamps began to flourish. Water covered the lower half of the state of Florida, and slowly drained into estuaries, supporting a dynamic salt water system. But as populations increased in Florida and the demand for farmland, timber and houses grew, the health of the Everglades was compromised. Developers rerouted parts of the Kissimmee River eliminating thousands of acres of floodplain and marshland. In the 1940's, the state government and the Army Corp of Engineers began to fix these mistakes. Scientists continually check the soil quality, water quantity and quality, water distribution, bottom sediments, and phosphorus levels to monitor the health of the Everglades. They also keep a close watch on plant distribution because some of the plants in the Everglades act as natural filters for excessive levels of phosphorus.